libutil: fix non-ASCII chars in URL encoding

Due to the cast to (unsigned int), the encoding appended broken
bytes padding. This is fixed here with a bitmask.

Fixes: https://git.lix.systems/lix-project/lix/issues/562
Change-Id: I0c93bd2b8c2f82df208d4693b7254544e3121dc3

src/libutil/url.cc

@@ -104,7 +104,7 @@ std::string percentEncode(std::string_view s, std::string_view keep)
             || keep.find(c) != std::string::npos)
             res += c;
         else
-            res += fmt("%%%02X", (unsigned int) c);
+            res += fmt("%%%02X", 0xff & (unsigned int) c);
     return res;
 }
 

tests/unit/libutil/url.cc

@@ -303,7 +303,6 @@ namespace nix {
         ASSERT_EQ(d, s);
     }
 
-
     /* ----------------------------------------------------------------------------
      * percentEncode
      * --------------------------------------------------------------------------*/
@@ -336,4 +335,10 @@ namespace nix {
         ASSERT_EQ(d, s);
     }
 
+    TEST(percentEncode, utf8Input) {
+        std::string s = percentEncode("ä");
+        std::string d = "%C3%A4";
+
+        ASSERT_EQ(d, s);
+    }
 }